The present work is directed toward determination of the heats of hydrogenation of large molecules, particularly, but not exclusively, molecules of biochemical or biomedical interest. The term "large molecule" is taken for these limited thermochemical purposes to mean molecules having a molecular weight of 100 or more, i.e., molecules for which hydrogen calorimetry is likely to yield more reliable results than traditional combustion calorimetry. Recent interest has been in the more common unsaturated and polyunsaturated fatty acids, methyl esters and triglycerides. Future work will be directed to less commonly-occurring lipids such as acids which are larger or smaller than the common C16 and C18 acids, unsaturated cholesteryl esters and others. The work has entailed and will continue to entail modification and redesigning of the calorimeter for greater sensitivity necessary to determine the best of hydrogenation of a small number of double bonds present in a large molecule which is otherwise inert to hydrogen calorimetry. At the present level of sensitivity, we are able to carry out a series of 9-16 determinations on a total of about 100 mg of lipid. The basic equation of hydrogen calorimetry is q equals n delta H where q is the measured heat, n is the number of moles present in the sample and delta H is the molar enthalpy change due to hydrogenation. Normally, one takes n moles of a pure sample, measures q and calculates delta H. However, the obverse procedure is also possible. Knowing delta H, one can measure q for a sample which is known or suspected not to be pure and calculate n. This analytical method is called enthalpimetry. We wish to pursue a systematic investigation of the enthalpimetry of naturally-occurring plant and animal lipid.